The present invention relates generally to ball screw actuators, and, more particularly, to an improved lock mechanism that is operatively associated with a ball screw actuator to selectively lock the screw shaft against further rotational or axial movement in the event of a failure of the primary nut or its attaching structure.
This invention relates generally to ball screw actuators for flight-critical aircraft control surfaces operating through a primary structural load path. The invention relates to a locking device that provides an emergency secondary structural load path that automatically carries the load in the event of certain failures in the primary load path.
The locking mechanism provides a positive connection between the screw shaft and the associated control surface in the event of a failure of the primary nut or its attaching structure. The locking mechanism is unloaded in normal operation, but locks the screw to the supporting member at the position of failure.
Previous attempts to solve this problem have typically used secondary nuts with free play on the screw which are deliberately configured to jam when loaded. However, some of these may have been unreliable. The present invention offers an improvement which is more complex, but, on the other hand, is highly reliable.
With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustration and not by way of limitation, the present invention provides an improvement in a device having a rotatable screw shaft and having a primary nut mounted on a movable member and threadedly engaging the screw shaft.
The improvement broadly comprises a standby locking mechanism (20) mounted on the member and operatively arranged to selectively lock the screw shaft (21) to the member (23) in the event of a failure of the primary nut (22). The standby locking mechanism includes a sensing mechanism (30) for sensing a failure of the primary nut, and at least one locking mechanism (29L or 29R) responsive to the sensing mechanism for selectively locking the screw shaft to the member upon a sensed failure of the primary nut. The standby locking mechanism may include a body (26) mounted on the movable member. It may further include an element (30) operatively engaging the screw shaft, and movably mounted on the body to sense axial movement of the screw shaft thread relative to the body from an initial position determined by the normal engagement of the screw shaft with the primary nut. Normally this element does not physically contact the shaft when the sensing mechanism is in its initial position. The improvement may further include a holding mechanism for releasably holding the element in its initial position. This holding mechanism may be a shear pin (32). However, in an alternative arrangement, it might well comprise a detent spring acting between the body and the element.
The locking mechanism includes at least one spring-biased member (35L, 35R) operatively arranged to selectively wedge between the body and the screw shaft to prevent further relative unidirectional movement therebetween. The locking mechanism may be prevented from wedging between the body and screw shaft by a trigger mechanism (33L, 33R) acting between the sensing mechanism and the locking mechanism.
Accordingly, the general object of the invention is to provide an improved ball screw actuator locking mechanism.
Another object is to provide an auxiliary locking mechanism for a ball screw, which does not interfere with, or frictionally impede, normal operation of the ball screw actuator.
Another object is to provide a ball screw actuator locking mechanism having means for sensing a failure of the primary nut, and for locking the ball screw against rotation in either direction in the event of a sensed failure of the primary nut.
Still another object is to provide a locking mechanism having an alternate structural load path to provide a positive connection between the screw shaft and the associated control surface in the event of a failure of the primary nut or its attaching structure.
These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.